Understanding of Active Sites and Interconversion of Pd and PdO during CH4 Oxidation

Pd-based catalysts are widely used in the oxidation of CH4 and have a significant impact on global warming. However, understanding their active sites remains controversial, because interconversion between Pd and PdO occurs consecutively during the reaction. Understanding the intrinsic active sites under reaction conditions is critical for developing highly active and selective catalysts. In this study, we demonstrated that partially oxidized palladium (PdOx) on the surface plays an important role for CH4 oxidation. Regardless of whether the initial state of Pd corresponds to oxides or metallic clusters, the topmost surface is PdOx, which is formed during CH4 oxidation. A quantitative analysis using CO titration, diffuse reflectance infrared Fourier-transform spectroscopy, X-ray diffraction, and scanning transmission electron microscopy demonstrated that a surface PdO layer was formed on top of the metallic Pd clusters during the CH4 oxidation reaction. Furthermore, the time-on-stream test of CH4 oxidation revealed that the presence of the PdO layer on top of the metallic Pd clusters improves the catalytic activity. Our periodic density functional theory (DFT) calculations with a PdOx slab and nanoparticle models aided the elucidation of the structure of the experimental PdO particles, as well as the experimental C-O bands. The DFT results also revealed the formation of a PdO layer on the metallic Pd clusters. This study helps achieve a fundamental understanding of the active sites of Pd and PdO for CH4 oxidation and provides insights into the development of active and durable Pd-based catalysts through molecular-level design.


Introduction
Natural gas, mainly consisting of methane, has received considerable attention as an alternative fuel for automobiles owing to its high energy efficiency. Furthermore, it is environmentally friendly, because it generates fewer undesirable by-products per unit energy when compared with other fossil fuels, such as gasoline and diesel. However, unburned CH 4 contributes significantly to global warming due to its~20 times higher greenhouse potential than that of carbon dioxide. Therefore, significant research efforts have been dedicated to eliminating CH 4 from exhaust gas.
Pd-supported catalysts are the most widely used catalysts for methane oxidation because of their high catalytic activity [1][2][3][4][5][6][7][8]. Despite significant research on catalytic active sites over the past decade, the understanding of the nature of the active sites on Pd-supported catalysts remains limited. Müller et al. proved the role of lattice oxygen on PdO nanoparticles (NPs) using labeled 18 O, suggesting that CH 4 oxidation follows the Mars-van Krevelen mechanism [9]. Iglesia et al. reported that the CH 4 oxidation reaction occurs via the redox cycle of the PdO surface [10,11]. Jang et al. demonstrated that Molecules 2023, 28, 1957 2 of 19 the PdO x species generated under the reaction conditions exhibited a linear correlation with CH 4 oxidation activity on Pd and Pt−Pd bimetallic catalysts [12]. On the contrary, Lyubovsky et al. suggested that metallic Pd is more active toward methane oxidation than PdO at high temperatures [13]. Xu et al. also proposed that CH 4 oxidation occurs on the metallic Pd surface via the Langmuir-Hinshelwood mechanism, and the stability of metallic Pd is important for CH 4 oxidation activity at low temperatures [14]. Hellman et al. reported that CH 4 easily dissociates from the partially reduced PdO(101) or metallic Pd rather than from PdO(100), using density function theory (DFT) calculations [15]. Furthermore, Zorn et al. reported that fully oxidized PdO did not participate in the reaction [16], and Mehar et al. reported that an intrinsic active site for Pd-based catalysts is not single-layer PdO(101) but multilayer PdO(101) [17].
Despite numerous studies on the active sites for CH 4 oxidation, the understanding of their nature remains limited. In this study, we investigated the behavior of Pd and PdO under the reaction conditions to establish the active sites for CH 4 oxidation. An investigation using CO titration, diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), and in situ X-ray diffraction (XRD) revealed that the initially formed PdO does not change during the CH 4 oxidation reaction. However, when it started from the metallic Pd cluster, a surface PdO layer gradually formed on the metallic Pd core during CH4 oxidation. Notably, regardless of whether the initial state of Pd corresponds to oxides or metallic clusters, the topmost surface is PdO x , suggesting that it is most probably the active site, and it is crucial for the activity and durability of CH 4 oxidation. To understand the structure of the experimental PdO particles, we identified the most stable positions for the O species on the surface and subsurface regions of the Pd(111) slab models at different coverages. In addition, we also interpretated the DRIFTS results of CO on PdO by employing Pd 79 nanoparticle models in our periodic density functional theory (DFT) calculations. This study helps to achieve a fundamental understanding of the active sites for CH 4 oxidation and provides insights into the development of active and durable Pd-based catalysts through molecular-level designs.

Characterization of Pd/Al 2 O 3 Catalysts
To understand the behavior of Pd and PdO under reaction conditions for CH 4 oxidation, we prepared 5.5 wt% Pd/Al 2 O 3 via hydrothermal aging (HTA) at 850 • C and characterized them using XRD and STEM after oxidative and reductive pretreatments at 500 • C, as shown in Figures 1 and 2. The oxidative pretreatment is denoted as "O x ", and the reductive pretreatment is denoted as "Red". The XRD patterns display the phase transformation of the support from γ-Al 2 Figure 1c,d exhibit characteristic peaks attributed to PdO at 2θ = 29.3 • , 33.9 • , and 41.9 • and 2θ = Pd at 40 • , respectively. The crystallite sizes of PdO and Pd estimated using the Scherrer equation are 15 nm and 30 nm, respectively. In contrast, the STEM images and particle size distribution displayed in Figure 2 clearly show significantly large PdO and Pd clusters of size 70 nm for oxidation and 53 nm for reduction. Datye et al. proposed that large metallic Pd clusters form polycrystalline metastable Pd/PdO structures during phase transformation [18,19]. Therefore, it can be interpreted that some of palladium during HTA is auto-reduced [20,21] and forms a polycrystalline Pd/PdO structure, which is consistent with the residual metallic Pd clusters shown in Figure 1c. The size discrepancy between the XRD and TEM results also elucidates that the Pd/PdO structure is polycrystalline with several grain boundaries, as shown in Supplementary Materials Figure S1. Based on the XRD and TEM results, the significant size discrepancy between PdO and Pd is caused by the polycrystalline nature of the Pd/PdO structure.

Catalytic Behaviors of Pd/Al2O3 Catalysts
To investigate the difference in effects of oxidative and reductiv

Catalytic Behaviors of Pd/Al2O3 Catalysts
To investigate the difference in effects of oxidative and reductiv catalytic behavior, we performed the CH4 oxidation reaction at 340 °C

Catalytic Behaviors of Pd/Al 2 O 3 Catalysts
To investigate the difference in effects of oxidative and reductive pretreatments on catalytic behavior, we performed the CH 4 oxidation reaction at 340 • C for 8 h, as shown in Figure 3. Significantly, the initial activity was comparable regardless of whether it was the oxidative or reductive pretreatment. Accordingly, it can be inferred that the active sites of Pd/Al 2 O 3 -O x and Pd/Al 2 O 3 -Red are most likely to be similar under the reaction conditions. Notably, the decrease in the catalytic activity of Pd/Al 2 O 3 -Red was slower than that of Pd/Al 2 O 3 -O x over 8 h of CH 4 oxidation. This indicates that the reductive pretreatment of Pd/Al 2 O 3 catalysts can improve their durability for CH 4 oxidation compared to oxidative pretreatment. To understand the reason for this observation, we characterized the bulk and surface properties under the reaction conditions through CO titration, in situ XRD, DRIFTS, and STEM. ecules 2023, 28, x FOR PEER REVIEW of water vapor [22][23][24][25][26][27][28][29][30][31], which forms during methane oxidation. Water thane for the active sites on the catalyst surface, since it can dissociate lytically active undercoordinated Pd sites, forming a spectator Pd-OH tive removal of strongly adsorbed OH species from the PdO surface peratures in excess of 500 °C [31,32].

Comparison of Pd and PdO under the Reaction Conditions
To investigate the transformation of metallic Pd and PdO durin used CO titration after the reaction had progressed for a certain period the oxygen to ascertain the relative proportion of PdO in the Pd/PdO c Figure 4. The reaction conditions for CH4 oxidation are the same as tho and CO titration was also performed at the same temperature after s with He. The initial analysis of Pd/Al2O3-Ox showed 100% of PdO, an during the course of the reaction over a period of 8 h. Therefore, there i during the CH4 oxidation reaction. However, interestingly, for Pd/A contents gradually increased as the reaction progressed and plateau after 3 h of reaction, which is consistent with the previous observation oxidized during the reaction [14]. Moreover, during CH4 oxidation at Pd clusters (<3 nm) completely oxidize to PdO( Figure 5). Based on t We should point out that, in the oxidized sample, the catalytic activity decreases from 40% to around 28% after 8 h of reaction. The issue with the deactivation of the Pd catalyst in the investigation reaction is complex and generally agreed to be related to the presence of water vapor [22][23][24][25][26][27][28][29][30][31], which forms during methane oxidation. Water competes with methane for the active sites on the catalyst surface, since it can dissociate and block the catalytically active undercoordinated Pd sites, forming a spectator Pd-OH species; the effective removal of strongly adsorbed OH species from the PdO surface occurs only at temperatures in excess of 500 • C [31,32].

Comparison of Pd and PdO under the Reaction Conditions
To investigate the transformation of metallic Pd and PdO during the reaction, we used CO titration after the reaction had progressed for a certain period of time to quantify the oxygen to ascertain the relative proportion of PdO in the Pd/PdO cluster, as shown in Figure 4. The reaction conditions for CH 4 oxidation are the same as those described above, and CO titration was also performed at the same temperature after sufficiently purging with He. The initial analysis of Pd/Al 2 O 3 -O x showed 100% of PdO, and it did not change Molecules 2023, 28, 1957 5 of 19 during the course of the reaction over a period of 8 h. Therefore, there is no change in PdO during the CH 4 oxidation reaction. However, interestingly, for Pd/Al 2 O 3 -Red, the PdO contents gradually increased as the reaction progressed and plateaued at~44% of PdO after 3 h of reaction, which is consistent with the previous observations that metallic Pd is oxidized during the reaction [14]. Moreover, during CH 4 oxidation at 340 • C, small-sized Pd clusters (<3 nm) completely oxidize to PdO( Figure 5). Based on these results, we inferred that metallic Pd is oxidized under the reaction conditions, forming a PdO layer on top of the metallic Pd clusters. The thickness of the PdO phase is~1.3-1.5 nm. Thus, in the case of smaller crystallites whose size is not sufficient to form the surface PdO layer on metallic Pd clusters, Pd is fully oxidized to PdO.      To confirm this inference, we performed in situ XRD experiments with different pretreatments under the reaction conditions ( Figure 6). Both catalysts exhibit common peaks, which are attributed to δ-Al2O3. XRD patterns for Pd/Al2O3-Red demonstrate only metallic Pd at 2θ = 40°, but Pd/Al2O3-Ox shows PdO at 2θ = 33.9° with a little metallic Pd at 2θ = 40°, as mentioned above (Figure 6a). After the reaction, the XRD patterns of Pd/Al2O3-Ox To confirm this inference, we performed in situ XRD experiments with different pretreatments under the reaction conditions ( Figure 6). Both catalysts exhibit common peaks, which are attributed to δ-Al 2 O 3 . XRD patterns for Pd/Al 2 O 3 -Red demonstrate only metallic Pd at 2θ = 40 • , but Pd/Al 2 O 3 -O x shows PdO at 2θ = 33.9 • with a little metallic Pd at 2θ = 40 • , as mentioned above (Figure 6a). After the reaction, the XRD patterns of Pd/Al 2 O 3 -O x did not change, which indicates that PdO does not change during the reaction, which is in agreement with the CO titration results. In contrast, for Pd/Al 2 O 3 -Red, a broad PdO phase was formed, and simultaneously, a slight decrease in the metallic Pd content was observed during the reaction (Figure 6b). Based on the CO titration results, the broad PdO phase is attributed to the~44% of PdO generated during the reaction. a representative band of PdOx at 2140 cm −1 [12] and a broad band at 2100-2090 cm −1 attributed to metallic Pd, which was partially reduced during CO adsorption [16]. The IR spectrum of Pd/Al2O3-Red collected before the reaction shows a distinctive peak at 2080 cm −1 , which is attributed to linear-bound CO, and two peaks at 1980 and 1940 cm −1 , which are attributed to bridge-bound CO on metallic Pd. The IR spectrum of Pd/Al2O3-Ox obtained after 8 h of reaction ( Figure 7a) shows a similar profile to that before the reaction, indicating that the initially formed PdO does not change during the reaction, which is in good agreement with the above results. However, on Pd/Al2O3-Red, partially oxidized PdOx at 2140 cm −1 appeared with the disappearance of bridge-bound CO on metallic Pd, yielding a similar spectrum to that of Pd/Al2O3-Ox. Notably, regardless of whether the initial state was metallic Pd or PdO, the catalysts showed the same topmost surface after the reaction.   Furthermore, to confirm the generation of the surface PdO layer, we characterized the surface compositions using DRIFTS after CO adsorption, as shown in Figure 7. The catalysts were tested over a period of 8 h of CH 4 oxidation under the same conditions in DRIFTS cells, and subsequently, IR spectra were collected after CO adsorption at RT following He purging. The IR spectrum of Pd/Al 2 O 3 -O x obtained before the reaction shows a representative band of PdO x at 2140 cm −1 [12] and a broad band at 2100-2090 cm −1 attributed to metallic Pd, which was partially reduced during CO adsorption [16]. The IR spectrum of Pd/Al 2 O 3 -Red collected before the reaction shows a distinctive peak at 2080 cm −1 , which is attributed to linear-bound CO, and two peaks at 1980 and 1940 cm −1 , which are attributed to bridge-bound CO on metallic Pd. The IR spectrum of Pd/Al 2 O 3 -O x obtained after 8 h of reaction ( Figure 7a) shows a similar profile to that before the reaction, indicating that the initially formed PdO does not change during the reaction, which is in good agreement with the above results. However, on Pd/Al 2 O 3 -Red, partially oxidized PdO x at 2140 cm −1 appeared with the disappearance of bridge-bound CO on metallic Pd, yielding a similar spectrum to that of Pd/Al 2 O 3 -O x . Notably, regardless of whether the initial state was metallic Pd or PdO, the catalysts showed the same topmost surface after the reaction.   In addition, as shown in Figure S2, the surface transformation from Pd to PdO occurred within 3 min of the reaction. Based on these results, we interpreted that the immediately generated topmost PdO x during the reaction is the active site for CH 4  In addition, as shown in Figure S2, the surface transformation from Pd to PdO occurred within 3 min of the reaction. Based on these results, we interpreted that the immediately generated topmost PdOx during the reaction is the active site for CH4 oxidation, and the similarity in the initial activity of Pd/Al2O3-Ox and Pd/Al2O3-Red in Figure 3   Based on the in situ XRD, DRIFTS, and STEM results, Pd/Al2O3-Red after CH4 oxidation reaction consists of a metallic Pd core with a surface PdO layer of 44%. This interpretation is consistent with the findings of Lee et al., who reported that the surface PdO layer on the metallic Pt core enhances the activity and long-term durability of CH4 oxidation [33]. Furthermore, it is also related to the report of Mehar et al. that multilayer PdO(101) the metallic Pt core enhances the activity and long-term durability of CH 4 oxidation [33]. Furthermore, it is also related to the report of Mehar et al. that multilayer PdO(101) on Pd(100) exhibited a better catalytic performance than single-layer PdO(101) [17]. Based on these results, we thought that the formation of the PdO layer on the metallic Pd core is the reason for the improved catalytic stability of Pd/Al 2 O 3 -Red shown in Figure 3. In Figure 6b, metallic Pd is dominant after the reaction, and someone can consider this as evidence that metallic Pd might be the active site. However, it is important to note that, regardless of whether it is the PdO or PdO/Pd core-shell structure, both catalysts have practically the same topmost active surface of PdO x .

DFT Modeling of the Interaction of O Species with Pd(111) Surface and Pd 79 Nanoparticle Models
Periodic DFT calculations were employed to clarify our experimental results. First, we compared the adsorption of one O atom on all available surfaces and subsurface positions of the Pd(111) model ( Figure 9). The most stable position was found to be the surface fcc, as the O species located initially at the br and oss positions also moved during the geometry optimization to fcc. The other three-fold hollow position, hcp, is less stable by 0.18 eV than the fcc position. The destabilization of the subsurface positions, tss and tss', is much higher at 1.32 and 1.95 eV, respectively, with respect to the fcc position.
Molecules 2023, 28, x FOR PEER REVIEW Figure 9. Notation of the site locations for the slab and NP models. Color coding: atoms from surface layer-orange; other Pd atoms-light blue.
To explore the coverage effect, we also considered models with high O surfa erage, where nine O centers are located on the available three-fold (fcc or hcp) sites. Similar to the situation with one O center, the model with nine O centers in f tions is more stable by 1.74 eV (0.19 per O center) than the model with nine hcp O Further, we considered models with high surface and subsurface O coverage. Th stable configuration was found with nine Ofcc and nine Otss, as it is more stable by (0.07 eV per O center) than the structure with nine O centers located at the fcc positions. The structure with the initial configuration 9Ohcp9Ooss converts into th stable one, 9Ofcc9Otss, during the geometry optimization. Thus, on the surface satur O Pd(111) and Pd79 models considered below, we positioned the surface and sub  Table 1). At low coverage, the process is strongly endothermic by 2.26 eV (Table 1) (Table 1).

DFT Modeling Adsorption of CO Species on Oxidized Pd(111) and Pd Nanoparticle
In our previous work, we reported CO adsorption on different positions of pristine Pd 79 palladium nanoparticles. Here, we considered CO adsorption on an oxidized palladium surface and nanoparticles. For CO adsorption on the oxidized palladium surface, we used Pd (111)   On the oxidized Pd 79 NP model, CO can be adsorbed on low coordinated sites (corners and edges), even when all 62 surface fcc sites are occupied by O species (Figure 11). The BE is −0.49 eV on the most accessible corner sites, while on the edge sites, the BE value is~0 eV, manifesting an unstable complex. The CO can be adsorbed at the terrace site if three neighboring O centers are removed from one of the (111)  cies slightly decreases the BE value from −0.49 to −0.37 eV. The highest BE value is fou for the Pd79(Ofcc)62(Otss)24/CO_top-edg model, −0.63 eV, although the BE value Pd79(Ofcc)62/CO_top-edg is ~0 eV. Surprisingly, the geometries of both carbonyl complex are the same-the Pd center is in a square planar configuration with respect to the neig boring O centers.

DFT Modeling Adsorption of CO Species on PdO(101) and PdO(100) Slab Models
The results obtained for both PdO(101) slab models are similar (Table 1, Figure 12); thus, only these obtained for the larger unit cell will be commented on in the text below. On the surface of the PdO(101) slab model, there are two types of Pd centers with coordination numbers three and four with respect to the closest O centers. We modeled CO adsorption on both surface positions, as the four-coordinated position is unstable, since the CO molecule migrates to a three-coordinated Pd center during geometry optimization. Indeed, in the most stable structure, PdO(101)/CO_top-3c, CO is adsorbed onto a three-coordinated surface Pd center, as the BE value, −1.49 eV, is similar to the corresponding value in the Pd 79 (O fcc ) 59 /CO_top-cen structure ( Table 2). We also optimized one more structure where CO is in the subsurface region, as the C atom interacts with two surface Pd centers and one subsurface Pd, while the O from the CO molecule interacts with one surface and one subsurface Pd centers. However, this complex is very unstable, as the BE is strongly positive at 3.18 eV. centers and one subsurface Pd, while the O from the CO molecule face and one subsurface Pd centers. However, this complex is ve strongly positive at 3.18 eV.
One complex is found when CO is adsorbed onto the Pd PdO(100)/CO_top-4c, where the CO interacts with a four-coordin The energy of CO in this case is only −0.04 eV.

Assignment of the Experimental C-O Vibrational Frequencies
In our previous study, we modeled CO adsorption at various Pd79 NP [34]. Our results have shown that, when CO is at the top top-edg, and top-cor), the calculated C-O vibrational frequencie sented also in Table 3 One complex is found when CO is adsorbed onto the PdO(100) surface model, PdO(100)/CO_top-4c, where the CO interacts with a four-coordinated surface Pd center. The energy of CO in this case is only −0.04 eV.

Assignment of the Experimental C-O Vibrational Frequencies
In our previous study, we modeled CO adsorption at various surface sites of pristine Pd 79 NP [34]. Our results have shown that, when CO is at the top coordination (topcen, top-edg, and top-cor), the calculated C-O vibrational frequencies, 2013-2023 cm −1 (presented also in Table 3), are systematically shifted to lower frequencies by~60 cm −1 than the experimental bands. Similarly, the calculated C-O vibrational mode for CO adsorption at the top position of the Pd(111) surface slab model, 2036 cm −1 , is 51 cm −1 lower than the experimental band at 2087 cm −1 . Table 3. Assignment of the C-O vibrational frequencies (in cm −1 ) for CO adsorption on the Pd and PdO x models and the experimental bands. The differences between the experimental and theoretical values (ν exp -ν calc ) are given in parentheses.  (Table 3). A similar increase of the C-O vibrational frequency from 2023 to 2051 cm −1 ( Table 2) with the covering of the surface of the nanoparticle is also found for the CO adsorption at terrace top-cen sites, despite the fact that the three closest O centers around the Pd atom, which interacts with the CO molecule, are removed. These DFT results show that the increasing of the Pd oxidation state leads to the increase in the vibrational frequencies due to the increase of the σ-donation and electrostatic interaction, and the decrease of the π-back donation interaction between the palladium and CO ligand [35]. This is also in line with our experimental findings that CO bands of metallic Pd are at 2068 and 2087 cm −1 , while, for the palladium, the oxide systems are at 2143 and 2160 cm −1 . Interestingly, when CO is adsorbed on the PdO(101) and Pd(100) surfaces, the calculated C-O vibrational frequencies, 2090 and 2131 cm −1 , are similar to the corresponding values in the oxidized Pd(111) and Pd 79 models, where the Pd center to which CO is bound is with coordination numbers 3 and 4, respectively. Hence, our calculated C-O vibrational frequencies for the structures with CN = 3 at 2083-2095 cm −1 (Table 3) can rationalize the experimental band at 2143 cm −1 , while the band at 2160 cm −1 can be rationalized by the CO on the edge of the oxidized structures, as well as PdO(100)/CO_top-4c, where the calculated C-O vibrational frequencies are at 2120-2121 and 2131 cm −1 (Table 3). In these structures, the coordination number of Pd to which CO is bound is 4. These results are in agreement with the previous study of Martin et al. [36], where an IR band at 2132 cm −1 is assigned to CO adsorption on the Pd(101) surface.

Catalyst Preparation
Catalysts were prepared via the incipient wetness impregnation method using an aqueous solution of the Pd precursor (Pd(NH 3 ) 4 (NO 3 ) 2 ) on Al 2 O 3 (PURALOX TH 100/150, Sasol) and dried at 110 • C for 30 min. The cycles of impregnation-drying were repeated 3 times to obtain a homogenous distribution of Pd. The metal loading of the prepared catalysts was 5.5 wt%. The prepared catalysts were calcined at 500 • C under 20% O 2 in He (total flow rate = 1000 mL/min) for 2 h and hydrothermally aged at 850 • C for 10 h under 5.7% H 2 O in the air. We chose 10 h of HTA at 850 • C to simulate the engine aging for 200 h in a vehicle. Pd/Al 2 O 3 -O x was prepared via calcination (20% O 2 /He) at 500 • C for 2 h, and Pd/Al 2 O 3 -Red was prepared via reduction (10% H 2 /He) at 500 • C for 1 h.

CH 4 Oxidation Reaction Tests and CO Titration
CH 4 oxidation reaction tests were performed in a quartz flow reactor using 50 mg of the catalyst at 340 • C. Prior to the catalytic tests, the catalysts were calcined at 500 • C for 1 h in 20% O 2 /He (flow rate = 60 mL/min). Reductive pretreatment was also conducted at 500 • C for 1 h in 10% H 2 /He (flow rate = 60 mL/min). CH 4 oxidation activity was measured as a function of reaction time with a mixture of 1% CH 4 and 10% O 2 in He balance (total flow rate = 60 mL/min). The effluent gases were passed through a cold trap (dry ice) to remove the water produced during the reaction and then analyzed via gas chromatography (GC, Agilent 7820A) using a Carboxen-1010 column and a thermal conductivity detector. CO titration was also conducted by repeated pulse of 10% CO/He (sample loop = 1 mL) at 340 • C, following He purging. The effluent gases were passed through a methanator with 40% Ni/Al 2 O 3 and analyzed using GC with flame ionization detector.

Catalyst Characterization
In situ DRIFTS experiments were performed using a Nicolet iS10 FTIR spectrometer equipped with a mercury cadmium telluride detector. The catalysts (13 mg) were loaded into a high-temperature reaction chamber (Harrick Scientific, NewYork, US) equipped with ZnSe windows, which were installed in a Praying Mantis diffuse reflectance accessory. Temperature ramping rate and gas flow rate were 10 • C/min and 60 mL/min, respectively. Loaded catalysts were pretreated using 10% O 2 /He gas at 380 • C for 30 min and cooled to room temperature (RT) by flushing with He for 30 min. Thereafter, the chamber was heated again to 380 • C under 10% H 2 /He, followed by He purging for 30 min and subsequent cooling under He flow. The background spectra were collected immediately after pretreatment. Afterward, CO adsorption using 0.2% CO/He was conducted. Each spectrum was obtained after He purging to remove the weakly bound CO and gas-phase CO. All spectra were collected using a resolution of 4 cm −1 , averaging 64 scans for every 50 s. XRD patterns were obtained using a high-resolution X-ray diffractometer (Rigaku, Smartlab, Tokyo, Japan) with Cu K α radiation (λ = 1.54 Å) in the step mode between 2θ values of 10 • and 80 • with a step size of 0.02 • /s. The tube voltage and current were 45 kV and 200 mA, respectively.
High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were obtained using JEM2100F (JEOL) operated at an accelerating voltage of 200 kV.

Computational Details and Models
Periodic DFT calculations were performed using the plane-wave-based Vienna ab initio simulation package (VASP) [37,38]. We employed the exchange-correlation functional suggested by Perdew and Wang (PW91) [39] with an energy cutoff of 415 eV. For a description of the interaction between atomic cores and electrons, we used the projector augmented wave (PAW) method [40,41]. We sampled the Brillouin zone using the scheme of Monkhorst and Pack [42], as the mesh is 5 × 5 × 1 k points for the Pd(111) slab models, while, for the nanoparticle models, as well as the slab PdO(101) and PdO(100) models, only the Γ point was used (see below). The Methfessel-Paxton technique [43] was invoked to accelerate convergence with a width of 0.1 eV, as the final total energies were extrapolated to zero smearing.
The binding energies of the adsorbed CO molecules E b were calculated as E b = −(E ad + E sub − E ad/sub ), as E ad is the total energy of the gas phase CO, E sub is the total energy of the Pd substrate (either or not containing pre-ad(ab)sorbed species), and E ad/sub is the total energy of the substrate with all the adsorbates in the optimized geometry. Thus, negative E b values indicate a favorable interaction, i.e., energy release during the adsorption process.
The shape of the employed Pd 79 nanoparticle (NP) model is a truncated octahedral. This structure exhibits eight (111) facets (each of them consisting of 12 surface Pd atoms) and six very small (100) facets (each of them consisting of four surface Pd atoms). The NP model is placed in a cubic cell with a lattice parameter of 2.0 nm.
We also employed an ideal Pd(111) surface model that was represented as five-layer slab repeated in a supercell geometry. The unit cell is (3 × 3); thus, the surface coverage of 1/9 is achieved when one species is adsorbed into the unit cell. The ad/absorbates were bound to one side of the model and were allowed to relax during geometry optimizations together with the three adjacent ("upper") layers of the substrate. The two "bottom" layers were kept fixed at the theoretical bulk-terminated geometry, where the Pd-Pd distance was 280 pm.
Other models that we employed were ideal PdO(101) and PdO(100) surfaces. In order to investigate the influence of the size of the unit cell in the former case, we considered two unit cells with a parallelepiped shape different in sizes. The available three-fold hollow Pd 3 adsorption sites for O and CO species on the Pd(111) slab and Pd 79 nanoparticle models were hcp (hexagonal close packed) or fcc (facecentered cubic) types ( Figure 1). The center of these sites was below a Pd atom in the first and second subsurface layers, respectively. The subsurface sites beneath hcp and fcc-tetrahedral and octahedral sites, respectively-were denoted as tss and oss. Other adsorption positions available on the surfaces of both considered models were a top and bridge (denoted as br). They were located above a surface Pd and a Pd-Pd bond, respectively. In order to distinguish the surface and subsurface sites in Pd 79 NP, we added an additional tag for the following site locations: (cen)-at the center of a (111) facet, (edg)-at the edge between two (111) facets, and (cor)-at the corner between two (111) and one (100) facet (see Figure 1). The subsurface tetrahedral sites just below a surface Pd atom were denoted as tss'.
The activation energy barriers for O penetration from the surface to the subsurface region were derived by scanning the potential energy profile along the line that connects the local minima of the initial (surface) and final (subsurface) positions of the O species. Thus, estimations of the transition states (TSs) were searched in a pointwise fashion along the path connecting the adsorption and absorption configurations, where the height of the carbon atom, defined with respect to the most distant frozen metal layer, was fixed, whereas all other degrees of freedom were allowed to fully relax. The normal mode analysis verified that TSs have one imaginary mode that corresponds to O surface↔subsurface movement.

Conclusions
In this study, we investigated the intrinsic active sites of the Pd/Al 2 O 3 catalyst for CH 4 oxidation, which interconverts between metallic Pd and PdO during the reaction. We used CO titration, in situ XRD, in situ DRIFTS, STEM, and periodic DFT calculations for the analysis. CO titration experiment and in situ XRD clearly showed that approximately 44% of PdO x was formed from metallic Pd during the CH 4 oxidation reaction. In situ DRIFTS revealed that the topmost PdO x surface is formed on the metallic Pd clusters during the reaction. STEM revealed the formation of the PdO layer on the metallic Pd cluster, and the DFT calculations also demonstrated that, in Pd nanoparticles, the migration of O from the surface to the subsurface region can be facilitated when the surface is O pre-covered, indicating the formation of surface PdO x species. Based on all these results, we concluded that, during the CH 4 oxidation reaction, regardless of whether the initial state is Pd or PdO, the topmost surface is composed of PdO x species, which is the catalytically relevant active phase. Furthermore, CH 4 oxidation activity profiles revealed that the surface PdO x layer on the Pd core structure improves the catalytic durability of CH 4 oxidation, which is in agreement with the findings of previous studies. We believe our study will advance the understanding about the active sites of Pd-based catalysts and provide insights into the development of durable Pd-based catalysts for CH 4 oxidation.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28041957/s1, Figure S1: High-magnification STEM images of Pd/Al 2 O 3 -Ox (red) and PdAl 2 O 3 -Red (blue); Figure S2: In situ DRIFTS spectra of adsorbed CO on Pd/Al 2 O 3 -O x (black) and Pd/Al 2 O 3 -Red after 3 min of CH 4 oxidation. DRIFTS spectra were recorded at 25 • C after He purging. The surface reconstruction from Pd to PdO immediately occurred within 3 min of the reaction. CH 4 oxidation condition-1% CH 4; 10% O 2 in He at 340 • C.